Unveiling the dynamic of nitrogen through migration and transformation patterns in the groundwater level fluctuation zone of a different hyporheic zone sediment

This study investigates the impact of water levels and soil texture on the migration and transformation of nitrate (NO3−-N) and ammonium (NH4+-N) within a soil column. The concentrations of NO3−-N gradually decreased from an initial concentration of 34.19 ± 0.86 mg/L to 14.33 ± 0.77 mg/L on day 70, exhibiting fluctuations and migration influenced by water levels and soil texture. Higher water levels were associated with decreased NO3−-N concentrations, while lower water levels resulted in increased concentrations. The retention and absorption capacity for NO3−-N were highest in fine sand soil, followed by medium sand and coarse sand, highlighting the significance of soil texture in nitrate movement and retention. The analysis of variance (ANOVA) confirmed statistically significant variations in pH, dissolve oxygen and oxidation–reduction potential across the soil columns (p < 0.05). Fluctuating water levels influenced the migration and transformation of NO3−-N, with distinct patterns observed in different soil textures. Water level fluctuations also impacted the migration and transformation of NH4+-N, with higher water levels associated with increased concentrations and lower water levels resulting in decreased concentrations. Among the soil types considered, medium sand exhibited the highest absorption capacity for NH4+-N. These findings underscore the significant roles of water levels, soil texture, and soil type in the migration, transformation, and absorption of nitrogen compounds within soil columns. The results contribute to a better understanding of nitrogen dynamics under varying water levels and environmental conditions, providing valuable insights into the patterns of nitrogen migration and transformation in small-scale soil column experiments.

www.nature.com/scientificreports/and transformation, affecting oxygen availability, microbial activities, and nitrification and denitrification processes 1,11 .Additionally, soil texture, characterized by particle size distribution, influences water-holding capacity, nutrient retention, and permeability 12,13 .Different soil textures exhibit varying absorption capacities for N compounds, consequently affecting their movement and retention in the soil 14 .Research on N migration and transformation in soil columns provides valuable insights into the complex dynamics of N compounds under different water levels and soil textures 15 .By monitoring the concentrations of NO 3 − N, NH 4 + -N, and NO 2 − -N at various depths within soil columns over a designated time period, researchers can investigate the response of N compounds to changing environmental conditions 16 .Previous studies have contributed to our understanding of the factors controlling N migration and transformation, providing insights for sustainable soil management and N pollution mitigation strategies 17,18 .In recent years, there has been a focus on studying the migration of pollutants, such as non-aqueous phase liquids (NAPL) and heavy metals, in porous media under groundwater level fluctuations [19][20][21] .Several studies have been conducted to investigate the migration of benzene series pollutants and crude oil in heterogeneous soil layers using indoor soil column experiments and monitoring wells.Groundwater level fluctuations have been found to impact the concentration of arsenic in groundwater in the Hanjiang Plain.The effect of water level fluctuations on the migration and transformation of iron has also been studied using a sand column experiment [22][23][24][25] .However, there has been relatively little research conducted on the migration of nitrogen under water level fluctuations.
Some studies have shown that water level fluctuations can promote the migration of NO 3 − N, leading to increased nitrate pollution in groundwater 26 .The content of NO 3 − N has been found to increase with nitrogen application and water level fluctuations, indicating a joint effect on the concentration of nitrate-nitrogen 27,28 .Another study demonstrated that shallow groundwater leaching can increase the content of nitrate-nitrogen and ammonium nitrogen in groundwater, with the depth of groundwater burial influencing the nitrogen content 29 .While there have been studies on the mechanism of nitrogen migration and transformation, research on the effects of groundwater level fluctuation on nitrogen migration and transformation is still limited.Existing studies have mainly focused on a single soil medium, which fails to capture the differences in nitrogen migration and transformation between different media under water level fluctuations.Additionally, the pollutants considered have primarily been NO 3 − N.However, there is a need for more systematic research on the mechanisms of nitrogen migration and transformation under groundwater level fluctuations.This research should consider different media and investigate additional pollutants beyond nitrate nitrogen.This study explore the migration and transformation of N compounds (NO 3 − -N, NH 4 + -N, and NO 2 − -N in soil columns under varying water levels and soil textures.Based on existing knowledge and observations, it is hypothesized that water levels and soil texture significantly influence the migration, transformation, and absorption of N compounds in soil systems.Changes in water levels are anticipated to impact the availability of oxygen, nitrification and denitrification processes, consequently affecting the concentrations of NO 3 − -N, NH 4 + -N, and NO 2 − -N.Furthermore, different soil textures are expected to exhibit varying absorption capacities for N compounds, leading to differences in their movement and retention in the soil. The specific objectives of this study are as follows: (1) Investigate the impact of water levels on the migration and transformation of NO 3 − -N, NH 4 + -N, and NO 2 − -N in soil columns; (2) Assess the absorption capacities of different soil textures (coarse sand, medium sand, and fine sand) for N compounds; (3) Analyze the temporal variations in N concentrations at different depths within soil columns and elucidate the influencing factors; and (4) Examine the sequence of N absorption rates for each soil texture under varying water levels.By addressing these objectives, The study aimed to unravel the intricate pattern of N movement and absorption in soil systems and contribute to our understanding of biogeochemical processes related to N dynamics but limited to the relationship between the N and other variables or sediment.
The findings of this study will provide a scientific basis for the remediation and treatment of soil, groundwater pollution as well as promoting sustainable soil and wastewater management practices.The results may also attract the attention of researchers, environmentalists, and agricultural professionals interested in understanding and addressing N pollution and its impacts on the environment.

Sample collection and testing
The study area selected for this research is the Weinan section of the Wei River Basin, which is the largest tributary of the Yellow River and serves as an important economic core area and ecological corridor in the Loess Plateau.The Weihe river is situated next to the Loess Plateau in the northern region and the North Qinling Mountains in the southern region.In Shaanxi, the main stream of the river spans a length of 502.4 km, with a drainage area covering 67,108 km 230 .The Weihe River basin encompasses a wide range of landscapes, climate patterns, and human activities, extending from its upper floodplain in Yangling to the lower reaches of Weinan.The geomorphological profile of the Weihe River basin is diverse.The upper floodplain in Yanglin is characterized by steep slopes and rugged terrain, through which the river flows in narrow valleys.As it continues towards Xingping, the river meanders through a broader floodplain with gentle slopes.In the areas of Xianyang and Xi'an, the basin expands further, and the river forms a large alluvial plain with fertile soils.Finally, the lower reaches of Weinan exhibit a well-developed floodplain.
In the sampling area, the sediments in the interaction zone consist of silt, coarse sand, and clay, which are arranged in alternating layers.For the purpose of this study, soil was chosen as the experimental medium due to its typicality and ease of sampling.The sediment samples collected from the HZ in the sampling area consisted of layers alternating between silt, coarse sand, and clay.The sampling procedure followed the methodology described in a previous study 31 .The collected soil samples were carefully placed on clean, dry geotextile to allow for natural air drying.Once the soil was fully dried, it was sieved using a 1 mm diameter sieve to remove any

Experimental device
The experiment utilized an organic glass column with an inner diameter of 12 cm and a height of 70 cm.The column had an opening at the top and was connected at the bottom to a pressure-measuring tube and a Markov bottle through a three-way valve.A peristaltic pump was positioned between the Markov bottle and the column to regulate the water level and simulate groundwater fluctuation (rise and drop).The pressure-measuring tube was securely fixed on the soil column.
The organic glass column had five sampling points from top to bottom, but only three points were used for this experiment.The sampling points were located at distances of 10 cm, 30 cm, and 50 cm from the bottom of the column, with adjacent point intervals of 10 cm, 20 cm, and 20 cm, respectively.The Rhizon SMS (Single Matrix Sampler) was employed at each sampling point.The manufacturer of the Rhizon SMS was Shanghai Saifu Bio-Tech Co., LTD. in Shanghai, China.The top and bottom of each column were covered with a layer of 5 cm quartz sand (diameter 2 ~ 3 mm) to maintain a constant rise and fall of the water level.The experimental device is depicted in Fig. 1.

Experimental design
The soil column experiment device, illustrated in Fig. 1, consisted of three pillars filled with the same quality of soil for different mediums.Each medium was filled to a height of 10 cm and compacted with a certain degree of strength.The soil medium was packed to the desired level, and the filling process was repeated until the column contained approximately 55 cm of soil medium.The soil solution sampler was placed at depths of 10 cm, 30 cm, and 50 cm within the column.
The upper opening of the column was sealed with plastic wrap to prevent moisture evaporation and later removed at the beginning of sample collection.Distilled water was introduced into the column from the Markov Table 1.Soil particle composition and parameters of the soil used.www.nature.com/scientificreports/bottle using a peristaltic pump, stabilizing the water level at a 25 cm scale.The initial water level was maintained for one week to allow for capillary effects within the soil column and to establish a new environment.A small pipe connected to the air was attached to the mouth of the Markov bottle.The water level was controlled by observing the pressure measuring tube on the side of the soil column.
For the experiment, a 1 L solution of potassium nitrate with a concentration of 300 mg/L and a 1 L solution of ammonium chloride with a concentration of 300 mg/L were prepared.The solutions were uniformly poured onto the top of the soil column, and the water level was adjusted using the peristaltic pump to stabilize the internal water at 25 cm for 1 week.After 1 week, water samples were collected at each sampling point to determine the initial N concentrations.
The water level underwent periodic changes, with a 1-week stabilization period following each change.The first periodic change occurred after twelve days, followed by changes after twenty-seven and thirty-one days, completing a total of three periods.The flow rate of the inlet and outlet water was controlled at 100-200 mL min −1 .At each water level change, the soil solution sampler was connected using a diaphragm vacuum pump.Water samples were collected at the four sampling points, and measurements were taken for pH, temperature, dissolved oxygen, NH 4 + -N, NO 3 − -N, and NO 2 − -N concentrations at each sampling point.To determine the pH value, a pH meter (Model: Micro600; Manufacturer: Palintest Co., Ltd., Beijing, China) was utilized.The temperature was measured using a thermometer.Dissolved oxygen was measured using a dissolved oxygen meter (Model: JPBJ-608; Manufacturer: Shanghai Yifen Scientific Instrument Co., Ltd., Shanghai, China).And the concentrations of NO 3 − -N, NO 2 − -N, and NH 4 + -N were determined using Flow Injection Analyzer (FIA) (San++skaler)Shanghai Yifen Scientific Instrument Co., Ltd., Shanghai, China).

Statistical analysis
In this research on the implications of water levels and soil texture on N migration and transformation in soil columns, Descriptive statistics were used to calculate the mean, standard deviation, and range of N concentrations at different soil depths and water levels, revealing overall patterns and variations.Analysis of Variance (ANOVA) was conducted to assess the statistical significance of differences in N concentrations among different water levels and soil textures.

Physicochemical parameters in the effluents
The study spanned a duration of 70 days and aimed to investigate the variations in temperature, pH, ORP, and DO content in different soil types (coarse sand, medium sand, and fine sand) under varying water levels.As depicted in Fig. 2a in the coarse sand soil column, the water level fluctuated between 5 and 45 cm throughout the experiment.But no clear trend observed in pH values ranged from 6.44 to 8.22.The DO levels varied between 1.2 and 3.5 mg/L, reaching a minimum of 1.2 mg/L on the 65th day.However, ORP values (103 mV to 245 mV) shows fluctuations throughout the experiment.
In the medium sand soil column (Fig. 2b), the pH values varied between 6.66 and 8.23.The DO levels fluctuated between 1.4 and 3.5 mg/L, with a peak of 3.4 mg/L on the 12th day.The pH and DO levels showed both decreasing and increasing trends over time while for the fine sand soil column (Fig. 2c), the pH values ranged from 6.44 to 8.12, displaying an increasing trend in the early stages and later fluctuating.The DO levels varied between 1.2 and 3.5 mg/L, with both increasing and decreasing trends observed and ORP values ranged from 110 to 289 mV, exhibiting fluctuations without a consistent pattern.Analysis of varience (ANOVA) confirmed the variation in physicochemical properties observed across the soil columns were statistically significant (p < 0.05).These findings demonstrate the dynamic nature of the physicochemical parameters in the different soil types, influenced by changes in water level over time.The pH, dissolved oxygen, and ORP exhibited varying patterns, indicating the complex interplay between the water level and the soil's chemical environment.These results provide valuable insights into the characteristics of the soil and its potential impact on various processes within the hyporheic zone.

Variation of NO 3 − -N change law
This study investigate the migration and transformation of NO 3 − -N within a soil column under different water levels and environmental conditions.To understand the movement and changes of NO 3 − -N, concentrations were monitored over a 70-day period at three distances within the soil column (10 cm, 30 cm, and 50 cm).The relationship between water levels and NO 3 ¯-N concentrations is shown in Fig. 3a-c.
In Column A (Coarse sand) as shown in Fig. 3a, the initial mean concentration of NO 3 − -N at the 10 cm distance was 34.19 ± 0.86 mg/L on day 0 and gradually decreased to 14.33 ± 0.77 mg/L on day 70.Similar decreasing trends were observed at the 35 cm and 45 cm distances, indicating the migration or transformation of NO 3 − -N within the column.At the 30 cm sampling point, fluctuations in NO 3 − -N concentration were observed with changing water levels.For instance, on day 0, with a water level of 45 cm, the concentration was 29.22 ± 0.75 mg/L.However, on day 15, with a lower water level of 15 cm, the concentration increased to 31.53 ± 1.08 mg/L.Subsequently, on day 30, with a higher water level of 35 cm, the concentration decreased to 24.89 ± 0.77 mg/L.Similar patterns were observed at the 50 cm distance, indicating the influence of water levels on NO 3 − -N concentrations.In Column B (Medium sand), the concentration of NO 3 − -N at the 10 cm distance started at 24.61 ± 0.59 mg/L on day 0, decreased to 7.65 ± 1.20 mg/L on day 36, and then slightly increased to 11.45 ± 1.45 mg/L on day 39, and further to 17.18 ± 0.99 mg/L on day 42.After day 42, the concentration fluctuated between 5.52 ± 1.05 mg/L and 17.18 mg/L due to water level fluctuations.Similar decreasing trends were observed at the 35 cm and 45 cm distances.Fluctuations in NO 3 − -N concentration were also observed at the 30 cm and 50 cm distances, corresponding to changes in water levels.
In Column C (Fine sand), the concentration of NO 3 − -N at the 10 cm distance started at 21.14 ± 0.24 mg/L on day 0 and fluctuated over time, reaching a peak of 22.20 ± 0.29 mg/L on day 3 and gradually decreasing to a minimum value of 16.29 ± 0.45 mg/L on day 70.Similar patterns of decreasing concentrations were observed at the 30 cm and 50 cm distances.Fluctuations in NO 3 − -N concentration were observed at the 30 cm and 50 cm distances, corresponding to changes in water levels.
The main theme highlighted in these findings is the variation of NO  on day 15, despite the water level dropping to 15 cm, the concentration increased to 6.88 ± 0.38 mg/L.On day 30, with a higher water level of 35 cm, the concentration decreased to 3.77 ± 0.23 mg/L.The lowest NH 4 + -N concentration was observed on day 55, with a water level of 45 cm, measuring 2.69 ± 0.17 mg/L.Finally, on day 70, with a water level of 15 cm, the NH 4 + -N concentration reached its lowest recorded value of 1.51 ± 0.13 mg/L.Similar trends were observed at sampling points 30 cm and 50 cm below the water level, suggesting a relationship between water levels and NH4 + -N concentrations in the coarse sand column.
Medium sand column (Column B): Fluctuations in NH 4 + -N concentrations were observed corresponding to different water levels over the 70-day period.At a sampling point 10 cm below the water level on day 0, with a water level of 45 cm, the NH 4 + -N concentration was 2.74 ± 0.43 mg/L.Surprisingly, on day 15, with a lower water level of 15 cm, the concentration increased to 3.90 ± 0.30 mg/L.On day 30, with a water level of 35 cm, the concentration decreased to 2.59 ± 0.26 mg/L.The lowest NH 4 + -N concentration was observed on day 70, with a water level of 15 cm, measuring 1.39 ± 0.30 mg/L.Similar patterns were observed at sampling points 30 cm and 50 cm below the water level, indicating variability in NH 4 + -N concentrations in response to different water levels in the medium sand column.
Fine sand column (Column C): NH 4 + -N concentrations were monitored at different sampling points and water levels over a 70-day period.At a sampling point located 10 cm below the water level on day 0, with a water level of 45 cm, the average NH 4 + -N concentration measured was 9.260 ± 0.06 mg/L on day 15, despite the water level being 15 cm lower, the mean NH 4 + -N concentration dropped to 11.660 ± 0.03 mg/L.Further observations showed a decrease in NH 4 + -N concentration to 6.790 ± 0.23 mg/L on day 30, with a water level of 35 cm.On day 55, with a water level of 45 cm, the NH4 + -N concentration decreased even further to 3.690 ± 0.13 mg/L.Finally, on day 70, with a water level of 35 cm, the NH 4 + -N concentration dropped significantly to 0.670 ± 0.20 mg/L.Similar trends were observed at a sampling point located 30 cm below the water level.
Transformation of NH 4 + -N NH 4 + -N into NO 3 − -N (nitrification): The mean values of NH 4 + -N at a depth of 50 cm showed distinct patterns corresponding to different water levels.On day 0, with a water level of 45 cm, the NH 4 + -N concentration was 0.074 ± 0.02 mg/L.On day 15, with a lower water level of 15 cm, the concentration increased significantly to 4.08 ± 0.14 mg/L.On day 30, with a water level of 35 cm, the concentration decreased to 0.38 ± 0.54 mg/L.On day 55, with a water level of 45 cm, the NH 4 + -N concentration increased to 0.89 ± 0.05 mg/L.Finally, on day 70, with a water level of 15 cm, the concentration decreased to 0.55 ± 0.27 mg/L.
Overall, the NH 4 + -N concentrations showed a decreasing trend throughout the experiment at each sampling point.However, the dynamics of NH Column C (Fine sand): The analysis of Column C, filled with fine sand, showed variations in NO 2 − -N concentrations at different depths and over time during the 70-day experiment.Initial concentrations at depths of 10 cm, 30 cm, and 50 cm were 0.94 mg/L, 0.87 mg/L, and 0.76 mg/L, respectively.Towards the end of the experiment, these concentrations decreased to 0.24 mg/L, 0.27 mg/L, and 0.25 mg/L at the same depths.Water level fluctuations were found to influence the migration and transformation of NO 2 − -N in the fine sand environment.Comparisons with other soil columns indicated that Column C had a moderate absorption capacity for NO 2 − -N, with concentrations higher than in Column A (coarse sand) but lower than in Column B (medium sand).The absorption capacity order for NO 2 − -N, from highest to lowest, was Column B > Column C > Column A.

Variation characteristics of physicochemical parameters in the experimental effluent
This study provides valuable insights into the variations of physicochemical parameters in the experimental effluent, including water level, pH, dissolved oxygen (DO), and oxidation-reduction potential (ORP).These variations are closely related to the migration and transformation of nitrogen (NO 3 − -N) within different sand types (coarse sand, medium sand, and fine sand) in the soil columns.Understanding these variations is crucial for comprehending the complex dynamics of nitrogen movement and its environmental implications.In the coarse sand soil column, the water level exhibited fluctuations throughout the study, which can significantly impact the movement of nitrogen within the soil.These fluctuations affect the saturation and aeration of the soil, which in turn influence the availability of oxygen and the processes of nitrification and denitrification 13,14,32 .These processes, in turn, affect the concentrations of NO 3 − -N in the effluent.The pH values in the coarse sand soil showed  are known to be influenced by pH levels 7 .Therefore, the variations in pH can indirectly affect the migration and transformation of NO 3 − -N within the soil column.The DO levels in the coarse sand soil displayed variations comparable to the medium and fine sand soils, reflecting changes in oxygen availability 33 .The presence of oxygen is crucial for the activity of nitrifying bacteria, which convert ammonium (NH 4 + ) into nitrate (NO 3 − ), and the absence of oxygen promotes denitrification, leading to the loss of NO 3 − -N 3 .The fluctuations in ORP in the coarse sand soil, similar to the medium sand soil, suggest variations in the redox reactions occurring in the soil 8 .Redox conditions play a significant role in determining the availability of electron acceptors and donors, affecting the transformation of nitrogen compounds.
In the medium sand soil column, the variations in pH, DO, and water level also contribute to the migration and transformation of NO 3 − -N.Fluctuations in pH, influenced by factors such as microbial activity and nutrient availability, can affect the activities of nitrifying and denitrifying bacteria, leading to changes in NO 3 − -N concentrations 32 .Additionally, changes in DO levels, influenced by microbial respiration, oxygen diffusion, and oxygen consumption by plants and organisms, can affect the rates of nitrification and denitrification, thereby influencing NO 3 − -N concentrations 23 .The fluctuations in water level impact the availability of water and soil moisture, which can influence microbial activity and the movement of NO 3 − -N within the soil.In the fine sand soil column, variations in pH, DO, and ORP also play a crucial role in the migration and transformation of NO 3 − -N.Changes in pH, influenced by factors such as acidic or alkaline compounds, microbial activity, and nutrient availability, can affect the activity of nitrifying and denitrifying bacteria, influencing the concentrations of NO 3 − -N.Fluctuations in DO levels, influenced by microbial respiration, organic matter decomposition, and oxygen diffusion, can impact the rates of nitrification and denitrification 7 .The variations in ORP reflect changes in the availability of electron acceptors and donors, affecting the redox reactions involved in nitrogen transformation.
Therefore, the variations in physicochemical parameters observed in the experimental effluent are closely related to the migration and transformation of NO 3 − -N within the soil columns of different sand types.Factors such as water level, pH, DO, and ORP influence the activities of microorganisms involved in nitrogen cycling, affecting the concentrations of NO 3 − -N 10 .Understanding these variations and their relationships with nitrogen dynamics is crucial for managing nutrient pollution, assessing soil health, and promoting sustainable agricultural practices 32 .Further research is needed to explore the specific mechanisms linking physicochemical parameters with nitrogen migration and transformation, allowing for more effective strategies for nitrogen management in agricultural and environmental contexts 17 .

Variation characteristics of inorganic N concentrations in the experimental effluent
The variation characteristics of inorganic nitrogen (N) concentrations in the experimental effluent provide valuable insights into the migration and transformation of NO 3 − -N, NH 4 + -N, and NO 2 − -N in different soil types.Fluctuations in concentration can be attributed to factors such as microbial activity, water flow, porosity, and transformations occurring within the column [26][27][28][29] .Soil texture plays a significant role in the movement and retention of water and nutrients, including NO 3 − -N.Medium sand, with its balanced composition of sand, silt, and clay particles, provides moderate water-holding capacity and nutrient retention.This allows water and NO 3 − -N to move more freely through the soil profile.Fine sand, with the largest particle sizes, results in faster drainage and reduced water and nutrient retention.These differences in soil texture affect the availability and movement of NO 3 − -N within the soil 10 .The sampling port above the water level showed no significant changes and was less affected by water level fluctuations.However, the sampling port below the water level was significantly affected by water level fluctuations, with the concentration of NO 3 − -N showing a significant decreasing trend during the water level fall stage.During the rising stage of water level fluctuation, the NO 3 − -N content in the soil solution greatly reduced, while during the dropping stage, the corresponding content significantly increased.This can be attributed to the decrease in dissolved oxygen content during the rising stage and the increase in dissolved oxygen concentration during the falling period 34 .The nitrification reaction is influenced by dissolved oxygen concentration, and a decrease in NO 3 − -N content leads to the accumulation of NH 4 + -N.The migration and transformation of NO 3 − -N in soil are influenced by factors such as soil water content, water flow movement state, dissolved oxygen content, and microbial community.Fluctuations in water level cause significant changes in soil water content, water flow movement state, and dissolved oxygen content in the soil section below the water level, leading to notable variations in NO 3 − -N content in the soil solution in that particular section.In the coarse sand soil, there was a rapid downward migration of NO 3 − -N above the water level due to its weak adsorption capacity for NO 3 − -N.Additionally, a strong denitrification effect above the water level resulted in low concentrations of NO 3 − -N.When the water level rose, there was a significant upward movement of NO 3 − -N below the water level, as the physical application caused by the rising water level outweighed the denitrification effect.Conversely, in fine sand, which has a strong adsorption capacity for nitrate N, there was minimal downward migration of NO 3 − -N above the water level in response to groundwater level fluctuations.The adsorption capacity of fine-sand soil hindered the migration of NO 3 − -N.During the rising stage of water level fluctuations, there was a significant increase in NH 4 + content in the soil solution.However, during the falling stage, there was a noticeable decline in NH 4 + -N concentration.The measured pH values and dissolved oxygen levels support the finding that lower dissolved oxygen concentrations contribute to higher NH 4 + -N concentrations in the soil solution 35 .Conversely, higher dissolved oxygen concentrations significantly reduce NH 4 + -N concentrations.This pattern can be explained by the rise in water level, which leads to a decrease in dissolved oxygen content and a transition from an aerobic stage to an anoxic stage.Denitrifying bacteria become more active, enhancing their ability to reduce compounds, leading to an increase in NH 4 + -N concentration and a decrease in NO 3 − -N concentration.Conversely, as the water level decreased, the dissolved oxygen content increased, promoting oxidation and the nitrification reaction.The effect of nitrification outweighed denitrification, leading to an increase in the mass concentration of NO 3 − -N.Furthermore, the soil's strong adsorption capacity for NH 4 + -N limits its accumulation in soil-free water, contributing to an overall downward trend in NH 4 + concentration.These findings align with previous research on variations in NH 4 + -N and NO 3 − -N concentrations in response to water level fluctuations.The fluctuations in NO 2 − -N concentration over time can be attributed to various factors, including microbial activity, water flow, and transformations occurring within the soil column.The concentrations of NO 2 − -N at different sampling points were influenced by the fluctuating water levels within the column.In general, the concentrations of nitrate N decreased over the course of the study, suggesting migration or transformation away from the sampling points.At the sampling points below the water level, the concentration of NO 2 − -N showed a significant decreasing trend during the rising stage of water level fluctuation.This may be attributed to a decrease in dissolved oxygen content during the rising stage, which impacts the nitrification reaction.In contrast, during the falling stage of water level fluctuation, there is an increase in dissolved oxygen concentration, resulting in an increase in nitrate N concentration.The rate of nitrification is accelerated as the dissolved oxygen concentration rises.
Overall, these findings imply that water level fluctuations can impact the migration and transformation of NO 2 − -N in a column filled with coarse sand.The fluctuating water levels likely influence the transport pathways, residence times, and interactions with the microbial community in the sediment.During the water level rise phase, there is an accumulation of NO 2 − -N, which could be attributed to the lower growth rate of nitrifying bacteria compared to denitrifying bacteria when dissolved oxygen is fully saturated.This leads to a higher rate of nitrification than denitrification, resulting in an increase in NO 2 − -N concentration.However, when the water level rises, the decrease in dissolved oxygen content affects the nitrification reaction 3 .In the water level decrease phase, the increase in dissolved oxygen concentration in the soil profile enhances the nitrification process, causing the nitrification reaction to produce H + ions.Over time, this leads to a weakly acidic solution.Studies have shown that pH values have a significant negative correlation with nitrate content but no significant correlation with ammonium and nitrite 33,36,37 .Therefore, a weakly acidic solution promotes the conversion of nitrite to nitrate, but it has no significant effect on ammonium and nitrite, resulting in no accumulation of nitrite and a gradual decrease in its content.Consequently, the findings demonstrate that the NO 2 − -N content fluctuates continuously throughout the experiment and eventually reaches a stable with obervable patterns.
The observed differences in absorption capacities among the soil columns can be attributed to variations in soil texture, porosity, and permeability.The medium sand in Column B creates more favorable conditions for NO 2 − -N absorption, while the coarse sand in Column A may have limited capacity due to its coarse texture.The fine sand in Column C falls in between, exhibiting intermediate absorption capabilities.These findings emphasize the importance of soil characteristics in determining the absorption capacity of NO 2 − -N.Understanding the variations in absorption capacities among different soil types is crucial for managing and mitigating N pollution and can inform agricultural and environmental practices to optimize nutrient retention in soil systems 10,11,31 .
The migration and transformation of NO 2 − -N in soil are influenced by factors such as soil water content, water flow movement state, dissolved oxygen content, and microbial community 33,35,[38][39][40] .The observed changes in soil water content, water flow movement state, and dissolved oxygen content below the water level due to fluctuations in the water level were found to be the main drivers behind the variations in nitrate N content in the soil solution in that particular section.It is important to note that the dynamics of nitrate-N migration and conversion can vary depending on the soil type.Coarse sand soil exhibited rapid downward migration of NO 2 − -N, while fine sand soil, with its strong adsorption capacity, hindered the migration of nitrate N above the water level in response to groundwater level fluctuations.Overall, this study highlights the complex relationship between water levels, soil characteristics, and N concentrations.

Conclusion
The concentrations of NO 3 − -N within the soil column exhibited fluctuations and migration influenced by water levels and soil texture.Higher water levels were associated with decreased nitrate N concentrations, while lower water levels resulted in increased concentrations.Fine sand soil demonstrated the highest retention and absorption capacity for nitrate N, followed by medium sand and coarse sand, emphasizing the significant role of soil texture in nitrate movement and retention.Variations patterns were observed in NO 3 − -N concentrations within the soil column.Fluctuating water levels influenced the migration and transformation of NO 3 − -N, with distinct patterns observed in coarse sand, medium sand, and fine sand.Water level fluctuations also had a significant impact on the migration and transformation of NH 4 + -N, with higher water levels associated with increased concentrations and lower water levels leading to decreased concentrations.Soil type influenced the absorption capacity of NH 4 + -N, with medium sand exhibiting the highest absorption capacity.Overall, the research findings highlight the crucial roles of water levels, soil texture, and soil type in the migration, transformation, and absorption of N compounds within soil columns.These conclusions align with the research objectives of understanding the dynamics of N migration and transformation under varying water levels and environmental conditions.

Data availability
The datasets generated and/or analysed during the current study are not publicly available In order to protect the privacy of my research data and that of the researchers involved, the original data will not be provided.The data are presented in various figures and tables in the manuscript but are available from the corresponding author on reasonable request.

Figure 1 .
Figure 1.Schematic diagram of the experimental setup.
3 − -N concentrations within the soil column under different water levels and distances.Both columns showed decreasing trends in NO 3 − -N concentrations over time, with fluctuations corresponding to changes in water levels.This indicates the migration or transformation of NO 3 − -N within the soil column and highlights the complex relationship between water levels and NO 3 − -N concentrations.Variation of NH 4 + -N change law The concentration of NH 4 + -N in different sand columns (coarse sand, medium sand) was monitored over a 70-day period to understand the relationship between water levels and NH 4 + -N concentrations.The following

Figure 2 .Figure 3 .
Figure 2. Changes in pH and DO and ORP in Different Soil types under varying water levels.The figures show the data collected from three columns containing coarse soil (a), medium sand (b) and fine sand (c), and the water level was varied from 45 to 5 cm in three cycles.Samples were collected every three days for the first two cycles and every five days for the last cycle.

4 +-
N were influenced by variations in the water level.As the water level rose, the NH 4 + -N concentration increased, and when the water level dropped, the NH 4 + -N concentration decreased.This trend of NH 4 + -N concentration.The trend of NH 4 + -N concentration showed an opposite pattern compared to the observed fluctuations in NO 3 − -N concentration in response to changes in water level.Comparing Figs. 3, 4, and 5, it is evident that the concentration of NO 2 − -N is much lower compared to nitrate nitrogen (NO 3 − -N) and NH 4 + -N.Column A (Coarse sand): Fig. 5a shows that the migration and transformation of NO 2 − -N in Column A, which was filled with coarse sand, were influenced by fluctuations in the water level.The water level in Column A fluctuated between 35 and 45 cm during the experiment, indicating changes in hydraulic conditions and nutrient transport within the column.NO 2 − -N concentrations varied spatially and temporally at different depths (10 cm, 30 cm, and 50 cm).The concentrations differed among the depths at each time point, emphasizing the importance of considering depth-dependent variations when analyzing NO 2 − -N migration and transformation Temporal variations in NO 2 − -N concentrations at each depth suggest dynamic processes of nitrogen transformation and transport influenced by factors like microbial activity, oxygen availability, and substrate availability.Column B (Medium sand): Fig. 5b focuses on Column B, which contained medium sand.It shows that the migration and transformation of NO 2 ¯-N in this column were influenced by water level fluctuations.The water level in Column B also fluctuated between 35 and 45 cm throughout the experiment.NO 2 − -N concentrations varied spatially and temporally at different depths (10 cm, 30 cm, and 50 cm), indicating dynamic nitrogen transformation and transport within the column.The depth-dependent variations highlight different NO 2 − -N concentrations at each depth at each time point, while the time-dependent variations demonstrate changes in NO 2 − -N concentrations over the course of the experiment.

Figure 4 . 4 +-
Figure 4. Changes in NH 4 + -Concentrations in different soil types under varying water levels.The figures show the data collected from three columns containing coarse soil (a), medium sand (b) and fine sand (c), and the water level was varied from 45 to 5 cm in three cycles.Samples were collected every three days for the first two cycles and every five days for the last cycle.

Figure 5 . 2 −-
Figure 5. Changes in NO 2 − -Concentrations in different soil types under varying water levels.The figures show the data collected from three columns containing coarse soil (a), medium sand (b) and fine sand (c), and the water level was varied from 45 to 5 cm in three cycles.Samples were collected every three days for the first two cycles and every five days for the last cycle.